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cmd/cgo: cgo error in generated wrappers points at unrelated source line #15836

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ALTree opened this issue May 25, 2016 · 5 comments

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@ALTree
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commented May 25, 2016

go version devel +095fbdc Wed May 25 16:49:02 2016 +0000 linux/amd64

The following program (sorry about the mpfr.h dependency, I wasn't able to reproduce it with a custom .h file):

package foo

/*
#cgo LDFLAGS: -lmpfr -lgmp
#include <gmp.h>
#include <mpfr.h>
*/
import "C"

func Bar() {
    _ = 1
    _ = 2
    _ = 3
    _ = 4
    _ = 5
    _ = 6
    _ = 7
    _ = 8
}

func Prec() C.mpfr_prec_t {
    return C.mpfr_get_prec(nil)
}

func Foo3() {
    _ = 1
    _ = 2
    _ = 3
}

func Foo2() {
    _ = 4
    _ = 5
    _ = 6
}

func Foo() {
    _ = 1
    _ = 2
    _ = 3
    _ = 4
    _ = 5
    _ = 6
    _ = 7
    _ = 8
    _ = 9
    _ = 10
    _ = 11
    // this isn't the line you're looking for
}

gives, on tip, an error that points to a bogus line in the source file. With tip:

# command-line-arguments
In file included from ./test.go:6:0:
./test.go: In function ‘_cgo_b5bec8c53dc8_Cfunc_mpfr_get_prec’:
./test.go:49:6: error: request for member ‘_mpfr_prec’ in something not a structure or union
  // this isn't the line you're looking for
      ^
./test.go:49:6: error: request for member ‘_mpfr_prec’ in something not a structure or union
  // this isn't the line you're looking for
      ^

with go1.6:

# command-line-arguments
In file included from ./test.go:6:0:
./test.go: In function ‘_cgo_b5bec8c53dc8_Cfunc_mpfr_get_prec’:
./test.go:41:23: error: request for member ‘_mpfr_prec’ in something not a structure or union
./test.go:41:23: error: request for member ‘_mpfr_prec’ in something not a structure or union

go1.6 is wrong too (it points to line 41), but at least it doesn't print it.

@ALTree ALTree changed the title cmd/go: cgo error points to the wrong line cmd/go: cgo error points to a wrong line May 25, 2016

@ianlancetaylor

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commented May 25, 2016

Note that the line is not being printed by any Go tool. It's being printed by the C compiler. I don't know why whether the line is printed or not changed from Go 1.6 to tip; using go build -x may show that we are passing different arguments to the compiler.

@ianlancetaylor ianlancetaylor changed the title cmd/go: cgo error points to a wrong line cmd/cgo: cgo error points to a wrong line May 25, 2016

@ianlancetaylor ianlancetaylor added this to the Go1.8 milestone May 25, 2016

@ALTree

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commented May 25, 2016

I don't know if error reporting for cgo code is supposed to be accurate when printing line numbers, so I'm not sure if this a bug or not; but right now we are printing completely bogus information, and it's rather confusing (even more confusing on tip, with a random line of code appearing in the error message).

More info about my env below.

Go 1.6:

$ go env
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/alberto/gocode"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GO15VENDOREXPERIMENT="1"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0"
CXX="g++"
CGO_ENABLED="1"

tip:

$ gotip env
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/alberto/gocode"
GORACE=""
GOROOT="/home/alberto/go"
GOTOOLDIR="/home/alberto/go/pkg/tool/linux_amd64"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build949309868=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"

the c compiler:

$ gcc --version
gcc (Debian 5.3.1-19) 5.3.1 20160509

-x output for go1.6:

$ go build -x test.go 
WORK=/tmp/go-build652767123
mkdir -p $WORK/command-line-arguments/_obj/
mkdir -p $WORK/
cd /home/alberto/Desktop/foo
CGO_LDFLAGS="-g" "-O2" "-lmpfr" "-lgmp" /usr/local/go/pkg/tool/linux_amd64/cgo -objdir $WORK/command-line-arguments/_obj/ -importpath command-line-arguments -- -I $WORK/command-line-arguments/_obj/ test.go
gcc -I . -fPIC -m64 -pthread -fmessage-length=0 -I $WORK/command-line-arguments/_obj/ -g -O2 -o $WORK/command-line-arguments/_obj/_cgo_main.o -c $WORK/command-line-arguments/_obj/_cgo_main.c
gcc -I . -fPIC -m64 -pthread -fmessage-length=0 -I $WORK/command-line-arguments/_obj/ -g -O2 -o $WORK/command-line-arguments/_obj/_cgo_export.o -c $WORK/command-line-arguments/_obj/_cgo_export.c
gcc -I . -fPIC -m64 -pthread -fmessage-length=0 -I $WORK/command-line-arguments/_obj/ -g -O2 -o $WORK/command-line-arguments/_obj/test.cgo2.o -c $WORK/command-line-arguments/_obj/test.cgo2.c
# command-line-arguments
In file included from ./test.go:6:0:
./test.go: In function ‘_cgo_b5bec8c53dc8_Cfunc_mpfr_get_prec’:
./test.go:41:23: error: request for member ‘_mpfr_prec’ in something not a structure or union
./test.go:41:23: error: request for member ‘_mpfr_prec’ in something not a structure or union

-x output for tip:

$ gotip build -x test.go 
WORK=/tmp/go-build055975085
mkdir -p $WORK/command-line-arguments/_obj/
mkdir -p $WORK/
cd /home/alberto/Desktop/foo
CGO_LDFLAGS="-g" "-O2" "-lmpfr" "-lgmp" /home/alberto/go/pkg/tool/linux_amd64/cgo -objdir $WORK/command-line-arguments/_obj/ -importpath command-line-arguments -- -I $WORK/command-line-arguments/_obj/ test.go
cd $WORK
gcc -fdebug-prefix-map=a=b -c trivial.c
gcc -gno-record-gcc-switches -c trivial.c
cd /home/alberto/Desktop/foo
gcc -I . -fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=$WORK=/tmp/go-build -gno-record-gcc-switches -I $WORK/command-line-arguments/_obj/ -g -O2 -o $WORK/command-line-arguments/_obj/_cgo_main.o -c $WORK/command-line-arguments/_obj/_cgo_main.c
gcc -I . -fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=$WORK=/tmp/go-build -gno-record-gcc-switches -I $WORK/command-line-arguments/_obj/ -g -O2 -o $WORK/command-line-arguments/_obj/_cgo_export.o -c $WORK/command-line-arguments/_obj/_cgo_export.c
gcc -I . -fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=$WORK=/tmp/go-build -gno-record-gcc-switches -I $WORK/command-line-arguments/_obj/ -g -O2 -o $WORK/command-line-arguments/_obj/test.cgo2.o -c $WORK/command-line-arguments/_obj/test.cgo2.c
# command-line-arguments
In file included from ./test.go:6:0:
./test.go: In function ‘_cgo_b5bec8c53dc8_Cfunc_mpfr_get_prec’:
./test.go:49:6: error: request for member ‘_mpfr_prec’ in something not a structure or union
  // this isn't the line you're looking for
      ^
./test.go:49:6: error: request for member ‘_mpfr_prec’ in something not a structure or union
  // this isn't the line you're looking for
      ^

@quentinmit quentinmit added the NeedsFix label Oct 5, 2016

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commented Oct 5, 2016

It seems like we must be passing the wrong line numbers to gcc; since gcc is printing errors in "test.go" and not in the generated C file, that means we're already providing line number info to gcc.

Docs at https://gcc.gnu.org/onlinedocs/cpp/Line-Control.html for how the line control directives are supposed to be formatted.

@rsc

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commented Oct 21, 2016

The line number is not really interesting here. The error is being reported in generated code, specifically the code that Go calls to wrap C's mpfr_get_prec. Maybe we should emit a #line that sets the file name to something like "".

Filed #17537 for the more serious problem that somehow mpfr_get_prec's wrapper is not being generated correctly.

@rsc rsc changed the title cmd/cgo: cgo error points to a wrong line cmd/cgo: cgo error in generated wrappers points at unrelated source line Oct 21, 2016

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commented Nov 3, 2016

CL https://golang.org/cl/32613 mentions this issue.

@gopherbot gopherbot closed this in b229022 Nov 3, 2016

@golang golang locked and limited conversation to collaborators Nov 3, 2017

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